Excitation vector generating apparatus and speech coding/decoding apparatus

Abstract

The total number of entries of an algebraic codebook is decreased by liming a random code vector generated from the algebraic codebook, and entries of a random codebook with a large number of pulses are assigned to a decreased portion. Further, the number of entries of the decreased portion is adaptively switched according to a mode.

1. A speech coding/decoding apparatus comprising:
 a partial algebraic codebook for generating excitation vectors, each comprised of two or more excitation pulses;  
 limiting means for limiting an interval between at least a pair of the excitation pulses to be relatively short, in an excitation vector among the excitation vectors; and  
 a random codebook used adaptively, corresponding to a size, with respect to the number of candidate vectors that may be generated, of the partial algebraic codebook.  
 
     
     
       2. The speech coding/decoding apparatus according to  claim 1 , wherein the limiting means controls the length of the interval using a relative relationship between a candidate position number (index) of each excitation pulse and changes the number of candidate relative-relationships for each pair of excitation pulses in accordance with whether the excitation vector corresponds to voiced speech or non-voiced speech. 
     
     
       3. The speech coding/decoding apparatus according to  claim 1 , wherein the size of the random codebook, with regard to the number of entries therein, is adaptively increased within a range of a decreased size of the partial algebraic codebook. 
     
     
       4. The speech coding/decoding apparatus according to  claim 1 , wherein the random codebook comprises a plurality of channels for generating excitation vectors comprised of excitation pulses and positions of the excitation pulses generated by the plurality of channels are limited so as not to overlap between the channels. 
     
     
       5. A base station apparatus comprising the speech coding/decoding apparatus according to  claim 1 . 
     
     
       6. A communication terminal apparatus comprising the speech coding/decoding apparatus according to  claim 1 .